With an ever-increasing data areal density in hard disk drives (HDD), the magneto-resistive (MR) sensor that is used as the read-back element in HDDs is required to have correspondingly better spatial resolution while at the same time achieving reasonable signal-to-noise ratio (SNR). FIG. 1 shows the structure of a generic TMR (tunneling-magneto-resistive) head which is the main MR sensor structure used in state-of-the-art HDD.
As seen in FIG. 1A, a generic TMR head has top and bottom reader shields 1 and 2 respectively, spaced distance 3 apart, hard bias (HB) magnets 5 on the sides and MR sensor stack 6 located between the reader shields. FIG. 1B shows conventional MR sensor stack 6 that includes free layer (FL) 8, tunneling barrier 9, reference layer 10, anti-parallel coupling layer 11 of Ru, pinned layer 12, and anti-ferromagnetic layer 13 beneath the pinned layer 12 to provide the pinned field on 12 and 10.
Between top shield 1 and free layer 8 is non-magnetic capping layer 7. The longitudinal magnetization of HB 5 provides a biasing magnetic field within sensor stack 6 to bias the magnetization 81 of free layer 8 in the cross-track direction. In today's hard disk drive, to further increase area data density, increased data linear density along both the down-track and cross-track directions is being developed. For higher track density, read heads with higher spatial resolution in the cross-track direction are required and smaller sensor sizes are needed. However, with smaller sensor size, magnetic noise gets worse as does sensor stability.
To overcome these magnetic noise and reduced stability problems, a stronger HB field is needed, but this also has the effect of making the sensor less sensitive. Furthermore, due to the smaller bit size within the medium, the field from the medium becomes smaller and so higher sensitivity sensors are required.
Thus, a trade-off exists between lower noise, better stability and higher signal. When solving this problem it is always beneficial to further increase the dR/R of the TMR film. This is, however, very hard to achieve in existing state-of-the-art TMR sensors. An Improved MR sensor design that can enhance the read-back signal without increasing noise and instability, are therefore needed.
A routine search of the prior art was performed with the following references of interest being found:
R. Olivier, and A. Satoru, “Magnetic tunnel junction read head using a hybrid, low-magnetization flux guide” see U.S. Pat. No. 6,519,124 B1 (2003). In U.S. Pat. No. 6,873,499, Lee et al. teach that a flux guide abuts the back edge of a read sensor. Dovek et al. in U.S. Pat. No. 6,239,955, show a flux guide on the back end of a MR sensor where the flux guide overlaps the lead and hard bias layers while Wu (in U.S. Pat. No. 7,170,721) discloses a flux guide on the side of a GMR element with permanent magnets surrounding the flux guide.